This invention generally relates to automobile suspension systems. More particularly, it pertains to an improved automobile suspension system with a shock absorber of multi-degree variable-damping-coefficient type between a sprung element and an unsprung element.
An automobile suspension system generally includes between a body (i.e., the sprung element) and wheels (i.e., the unsprung elements) shock absorbers to damp up-and-down vibration of the wheels. Many types of shock absorbers are known in the art. For example, there is known a shock absorber of bi-degree variable-damping-coefficient type so that the damping coefficient can vary from a greater degree to a lesser degree, and vice versa. A shock absorber of multi-degree or infinitely variable-damping-coefficient type is likewise known.
The basic operation principles of such a shock absorber are that its damping coefficient is changed and controlled so as to harmonize an actual damping force, generated by the shock absorber, with a target damping force (i.e., the skyhook damper force) that causes no sprung vertical motion. Japanese Patent Application Laying Open Gazette No. 60-248419 discloses a shock absorber control method which checks for the agreement between a sign of the relative displacement between the sprung element and the unsprung element and another sign of the differentiated value thereof, (i.e., the relative velocity between the sprung element and the unsprung element). At the time when these two signs agree with each other, the damping coefficient of the shock absorber increases, which results in the increase of the damping force generated by the shock absorber. On the other hand, at the time when the two signs disagree with each other, the damping coefficient of the shock absorber decreases, which results in the decrease of the damping force generated by the shock absorber.
Such a control method described above employs a well known technique that a dead band zone is provided so as to prevent the damping coefficient of the shock absorber from being changed many times against displacement taking place in the vicinity of a neutral position. An example of this is disclosed in Japanese Utility Model Application Laying Open Gazette No. 63-40213. In accordance with this prior art technique, a dead band zone is provided with respect to the relative displacement between the sprung element and the unsprung element in controlling and changing the damping coefficient based on the foregoing agreement or disagreement diagnosis between the two signs. The damping coefficient of the shock absorber always keeps remaining at a lesser degree within the dead band zone. Further, the width of the dead band zone gets narrow when sprung element input such as a steering angle and a steering angle velocity is great so that rolling motion of the automobile can be controlled.
Generally, individual damping coefficients of different degrees, regardless of their magnitude, can be considered to have the same dead band zone in view of facilitating controlling.
This, however, presents the following drawbacks. That is, the decrease of the damping coefficient causes the absolute value of the damping force to decrease so that the change of the damping coefficient is carried out more than necessary even against the input of low external forces, when the damping coefficient is of a lesser degree. This results in the occurrence of chattering. Besides, when a rapid, severe external force works on the automobile, such as when traveling over a bump on the road surface, the change to a lesser degree damping coefficient to cope with such an external force is delayed due to the presence of the dead band zone. This results in the lack of damping force.